Radiation curable polyurethane resins are widely used in radiation curable coating compositions for various substrates. Because polyurethane resins are generally flammable, flame retardants must be incorporated in the resins for many applications. However, traditional flame retardants typically contain halogens or require halogens in their manufacture. For example, flame retardant modified ultraviolet curable urethane acrylate resins with monomers containing bromine and phosphorus have been reported by W. Guo in the Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 30, pp. 819-827 (1992).
To avoid the toxic and corrosive combustion products associated with the use of halogens, research efforts are presently directed toward developing radiation curable, halogen-free flame retardant coatings. It has been reported that a phosphonate compound having a polymerizable vinyl group can be attached by radiation curing to an acrylated, aliphatic polyester urethane. While the compound is an improvement over the halogen-containing flame retardants, the manufacture of the compound requires an acyl halide to form the vinyl group and produces hydrogen chloride as a manufacturing by-product. Both of these halide compounds present handling and disposal problems.
A need exists for a radiation curable composition that contains no halogens, is manufactured without halide compounds and produces a halogen-free, flame retardant polyurethane resin polymer. Preferably, the radiation curable composition would produce a flame retardant polymer which resists leaching by organic chemicals such as acetone and exhibits relatively low thermal weight loss. It would also be desirable to produce a flame retardant polymer that is optically clear, which excludes the use of inorganic fillers such as hydrated alumina, magnesium hydroxide or antimony oxide.
One application for such a composition is the matrix material in optical fiber plenum cables. The flammability requirements for plenum cables are very stringent and difficult to meet without a flameoretardant matrix material. A matrix is a composition that covers or coats a group of optical fibers. Typically, the group consists of about 4 to about 8 optical fibers; however, the group may consist of more than 8 or less than 4 optical fibers. The matrix is used to help bond the optical fibers together and protect the fibers from environmental stress even though the optical fibers may already be individually coated with various coatings. A clear, non-halogenated, radiation curable matrix material provides for easy visibility of the optical fibers, low corrosivity of combustion products, and fast line speeds.